Research

An introduction to Raman scattering

Raman scattering is one of the key phenomena being used by the Quantum Technologies group; it is an inelastic light scattering phenomenon. When light scatters from a suitable Raman-active medium, new frequencies appear in the spectrum: red-shifted frequencies are termed ‘Stokes’ frequencies, while blue-shifted frequencies are termed ‘anti-Stokes’ frequencies (a). The energy donated to, or gained from, the medium is typically in the form of a non-propagating excitation such as a vibrational excitation (optical phonon) or rotational excitation, although electronic and spin-flip transitions are also observed.

Raman scattering: (a) a pump pulse propagates through a Raman medium, generating red-shifted Stokes sidebands and blue-shifted anti-Stokes sidebands by inelastic scattering. (b) In a Stokes transition a pump photon is annihilated as the system transfers from state ‘g’ to ‘e’, with the remaining energy carried away in the red-shifted Stokes photon. (c) In an anti-Stokes transition a pump photon is annihilated as the system transfers from state ‘e’ to ‘g’, with the remaining energy carried away in the blue-shifted anti-Stokes photon.

In a Stokes scattering event (b), on interacting with the Raman-active system, the input photon is annihilated, and the system makes the transition from the lower energy level ‘g’ to the upper level ‘e’, with the excess energy carried away by the red-shifted Stokes photon. An anti-Stokes scattering event (c) is the same in all respects except the system makes the transition from the upper energy level ‘e’ to the lower energy level ‘g’, donating energy to the field.

The described single-photon events are cases of spontaneous Raman scattering, which is a purely quantum process, involving no other photons. However, in the presence of intense laser light, Raman scattering can become a stimulated process, with Stokes photons stimulating the scattering of further Stokes photons from an intense input “pump” beam. Stimulated Raman scattering is a tremendously useful phenomenon in modern science. It is frequently used in precision spectroscopy, in nonlinear optics for frequency-shifting, and also as a method of control, allowing scientists to effect control of material dynamics by careful application of coherent light.